Systems And Processes For Bifacial Collection And Tandem Junctions Using A Thin-Film Photovoltaic Device

ABSTRACT

A thin-film photovoltaic device includes a semi-transparent back contact layer. The semi-transparent back contact layer includes a semi-transparent contact layer and a semi-transparent contact interface layer. The thin-film photovoltaic device may be formed in a substrate or superstrate configuration. A tandem thin-film photovoltaic device includes a semi-transparent interconnect layer. The semi-transparent interconnect layer includes a semi-transparent contact layer and a semi-transparent contact interface layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/858,010 filed Sep. 19, 2007, which claims benefit of priority to U.S.Provisional Patent Application Ser. No. 60/845,705 filed Sep. 19, 2006.Each of the above-mentioned applications is incorporated herein byreference.

U.S. GOVERNMENT RIGHTS

This invention was made with Government support under NationalAeronautics and Space Administration Grant No. NNC05CA41C. TheGovernment has certain rights in this invention.

BACKGROUND

Photovoltaic (“PV”) devices generally consist of one or more activephotovoltaic materials capable of generating an electric potential uponexposure to light, and electrical contacts constructed on a suitablesubstrate that are used to draw off electric current resulting fromirradiation of the active PV material. PV devices are generally rigid,either because the active PV material itself is rigid, or because thesubstrate or other components of the device are inflexible. For example,glass, which is relatively inflexible, is frequently used as a substratein thin film photovoltaic (“TFPV”) devices because of its strength,durability, tolerance to high processing temperatures and desirableoptical properties.

TFPV devices are commonly distinguished from their thickersingle-crystal PV counterparts in their ability to absorb light inrelatively thin layers, and their ability to function well whenfabricated using low-cost deposition techniques, and upon a variety oflow-cost, lightweight and flexible substrates. Thus, TFPV devices arebeing considered for a variety of applications where weight andflexibility are important, such as for space satellites andhigh-altitude airships.

TFPV devices commonly include a solar absorber layer formed of a GroupII-VI material, a Group I-III-VI.sub.2 material, or a Group III-Vmaterial. However, a solar absorber layer can be formed of othermaterials. The term Group II-VI material refers to a compound having aphotovoltaic effect that is formed from at least one element from eachof groups II and VI of the periodic table. In the context of thisdisclosure, Group II elements include Zinc, Cadmium, Mercury, andMagnesium and Group VI elements include Sulfur, Selenium, and Tellurium.The term Group I-III-VI.sub.2 material refers to a compound having aphotovoltaic effect that is formed of at least one element from each ofgroups I, III, and VI of the periodic table, where there are two atomsof the group VI element for every one atom of the group I and IIIelements. In the context of this disclosure, Group I elements includeCopper, Silver, and Gold, and Group III elements include Boron,Aluminum, Gallium, Indium, and Thallium. The term Group III-V materialrefers to a compound having a photovoltaic effect that is formed from atleast one element from each of groups III and V of the periodic table.In the context of this disclosure, Group V elements include Nitrogen,Phosphorous, Arsenic, Antimony, and Bismuth.

Prior art TFPV devices with flexible substrates typically use metal foilor polyimide substrates. Metal foil substrates are capable ofwithstanding the high-temperatures and harsh thin-film depositionconditions needed for the highest efficiency TFPV devices, however, theyare relatively heavy and are opaque. The latter characteristic does notallow for bifacial or backside visible light collection from reflectedlight sources, such as albedo light from either the moon or earth. Nordoes this characteristic allow for transmission of undesirable infra-red(“IR”) light through the TFPV device; unused and untransmittedsub-bandgap light increases the operating temperature of the TFPV deviceand thereby decreases its efficiency. In one example, increased TFPVdevice operating temperature decreases efficiency by as much as 20%.Finally, opaque substrates do not allow for devices fabricated in thesuperstrate configuration, where the highest intensity visible lightfirst passes through the substrate. Polyimide substrates aresemi-transparent to IR light, however, they are only partiallytransparent to visible light or capable of withstanding the highesttemperature thin-film deposition conditions required for certainCuInGaSe₂ (“CIGS”) based devices.

Attempts to provide PV devices on flexible and semi-transparentsubstrates are disclosed in U.S. Pat. No. 4,816,324, wheretetrafluoroethylene-perfluoroalkoxy resin is used as a substrate for thePV device. However, tetrafluoroethylene-perfluoroalkoxy resin cannotwithstand processing temperatures higher than 200-250° C., and thereforecannot be used for fabrication of high-efficiency TFPV devices thatutilize Group II-VI and Group I-III-VI.sub.2 light-absorber materials,such as Cadmium Telluride (CdTe) and Copper Indium Gallium Di-Selenide(CIGS), respectively, since these materials require significantly higherprocessing temperatures.

Another example of a substrate that is lightweight, flexible, and thatcomprises materials such as silicon or silicone resin that aresemi-transparent to visible light is described in U.S. ProvisionalPatent Application Ser. No. 60/792,852, and U.S. Non-Provisional patentapplication Ser. No. 11/737,119, each of which are incorporated hereinby reference. This particular substrate is capable of withstandingprocessing temperatures up to 600° C., thereby enabling use ofhigh-efficiency CIGS and CdTe materials in fabricating TFPV devices.However, to enable bifacial light collection, both the top and bottomcontacts to the TFPV device must be at least semi-transparent to visiblelight.

To increase efficiency of TFPV devices through bifacial collection,semi-transparent substrates and/or semi-transparent back contacts areneeded. For example, a semi-transparent back contact using a thin metalfilm (e.g., Cu) followed by a transparent conducting oxide (TCO) (e.g.,Indium Tin Oxide) has been used with CdTe thin film Group II-VIsemiconductor materials grown in a superstrate configuration on heavy,rigid glass substrates, as disclosed in a paper titled “Analysis of aTransparent Cu/ITO Contact and Heat Treatments on CdTe/CdS Solar Cells”by R. Birkmire, S. Hegedus, B. McCandless, J. Phillips and W. Shafarman[Proc. 19^(th) IEEE PVSC (1987), p 967]. However, a thin Cu layer wouldbe difficult to implement with thin-film devices grown in a substrateconfiguration, because the back contact layer is deposited first and maybe damaged and diffuse into other layers during the subsequentprocessing required for the solar absorber material. Application of thesolar absorber material typically includes high heat, vacuum, and use ofreactive elements such Se or S. Thus, transparent back contact materialsutilized for superstrate configuration cannot necessarily be used forsubstrate configuration.

Semi-transparent back contact grids have been used along with a highlydoped back semiconductor in Group II-VI solar cells materials (e.g.,CdTe, ZnTe) in the superstrate configuration on heavy, rigid glasssubstrates for mechanically stacked four-terminal tandem TFPVs, asdisclosed in a paper titled “Polycrystalline CdTe on CuInSe₂ CascadedSolar Cells,” by P. Meyers, C. Liu, L. Russell, V. Ramanathan, R.Birkmire, B. McCandless and J. Phillips [Proc. 20^(th) IEEE PVSC (1988),p 1448].

Solar absorbing layers of typical CIGS TFPV devices are p-type and withback contacts formed by intimate connection to thick, opaque metals suchas Mo, Ni, or Au that form a low resistance Schottky barrier contact.Thus, these typical high-performance back contacts do not enable visiblelight to pass through.

It has not been possible to fabricate TFPV devices with semi-transparentback contacts and acceptable performance where the TFPV device is basedupon high-bandgap (greater than 1.4 eV) Group I-III-VI.sub.2 materials.For example, when standard semi-transparent TCOs are used without aninterface layer as the back contact for wide-bandgap CuInGaSe₂ (CIGS)solar absorbing material, low efficiency (e.g., less than 4% efficient)devices result. However, the same back contact layer used withlow-bandgap (less than 1.2 eV) CIGS solar absorbing material produceshigh-efficiency devices (e.g., greater than 10% efficient). Thus,transparent back contact materials utilized for low-bandgap solarabsorber materials cannot necessarily be used for wide-bandgap solarabsorber materials, as needed for a transparent interconnect inmonolithic two-terminal tandem devices.

Monolithic two-terminal tandem devices in the substrate configurationbased on crystalline III-V materials and amorphous/microcrystalline Sihave been fabricated and commercially sold. In the case of thecrystalline III-V bottom cell, the tandem device is fabricated attemperatures greater than 700° C. using expensive deposition equipmentfor controlled crystal growth that is not amenable to very large areadepositions. Thus this technology cannot be reasonably applied torelatively inexpensive large-area depositions using polycrystallinethin-films on low cost and/or flexible substrates. Furthermore, thetransparent back contact design concepts of crystalline andamorphous/microcrystalline silicon devices such as tunnel junctioninterconnects cannot be readily transferred to devices based onpolycrystalline CIS and related alloys. Such difficulty in transferringdesign concepts is due to difficulty in achieving tightly controlledspatial positioning required by tunnel junctions through doping anddiffusion of impurities when in the presence of grain boundaries, whichact as conduits for diffusion. In addition, very high levels of dopingare difficult to achieve in CIS based alloy materials without alsocreating compensating defects. Thus, other device designs/structures maybe preferred.

SUMMARY

A thin-film photovoltaic device includes a semi-transparent substratefor supporting the thin-film photovoltaic device. A semi-transparentback contact layer is disposed on the semi-transparent substrate. Thesemi-transparent back contact layer includes a semi-transparent contactlayer disposed on the semi-transparent substrate, and a semi-transparentcontact interface layer including a Cu(X)Te₂ material disposed on thesemi-transparent contact layer. X is at least one of In, Ga, and Al. Asolar absorber layer is disposed on the semi-transparent back contactlayer, and the solar absorber layer includes one of a GroupI-III-VI.sub.2 material and a Group II-VI material. A heterojunctionpartner layer disposed on the solar absorber layer, and a top contactlayer is disposed on the heterojunction partner layer.

A thin-film photovoltaic device includes a semi-transparent substratefor supporting the thin-film photovoltaic device and a top contact layerdisposed on the semi-transparent substrate. A heterojunction partnerlayer is disposed on the top contact layer, and a solar absorber layeris disposed on the heterojunction partner layer. The solar absorberlayer includes one of a Group I-III-VI.sub.2 material and a Group II-VImaterial. A semi-transparent back contact layer is disposed on the solarabsorber layer. The semi-transparent back contact layer includes asemi-transparent contact interface layer including a Cu(X)Te₂ materialdisposed on the solar absorber layer and a semi-transparent contactlayer disposed on the semi-transparent contact interface layer. X is atleast one of In, Ga, and Al.

A tandem thin-film photovoltaic device includes a substrate forsupporting the device. A back contact layer is disposed on thesubstrate, and a bottom solar absorber layer is disposed on the backcontact layer. A bottom heterojunction partner layer is disposed on thebottom solar absorber layer. A semi-transparent interconnect layerincludes a semi-transparent contact layer disposed on the bottomheterojunction partner layer and a semi-transparent contact interfacelayer disposed on the semi-transparent contact layer. Thesemi-transparent contact interface layer includes a Cu(X)Te₂ material,where X is at least one of In, Ga, and Al. A top solar absorber layer isdisposed on the semi-transparent interconnect layer, where the top solarabsorber layer includes one of a Group I-III-VI.sub.2 material and aGroup II-VI material. A top heterojunction partner layer is disposed onthe top solar absorber layer, and a top contact layer is disposed on thetop heterojunction partner layer.

A thin-film photovoltaic device includes a substrate for supporting thedevice. The substrate includes at least one of silicone, reinforcedsilicone, reinforced silicone resin, and silicone coated metal foil. Asemi-transparent back contact layer is disposed on the substrate. Thesemi-transparent back contact layer includes a semi-transparent contactlayer disposed on the substrate and a semi-transparent contact interfacelayer disposed on the semi-transparent contact layer. A solar absorberlayer is disposed on the semi-transparent back contact layer, and aheterojunction partner layer is disposed on the solar absorber layer. Atop contact layer is disposed on the heterojunction partner layer.

A thin-film photovoltaic device includes a semi-transparent siliconesubstrate for supporting the device. A top contact layer is disposed onthe semi-transparent silicone substrate, and a heterojunction partnerlayer is disposed on the top contact layer. A solar absorber layer isdisposed on the heterojunction partner layer, and a semi-transparentback contact layer is disposed on the solar absorber layer. Thesemi-transparent back contact layer includes a semi-transparent contactinterface layer disposed on the solar absorber layer and asemi-transparent contact layer disposed on the semi-transparent contactinterface layer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a cross-sectional schematic view of a structure forming oneTFPV device in substrate configuration, in accordance with anembodiment.

FIG. 2 shows a cross-sectional schematic view of one monolithic tandemTFPV device, in accordance with an embodiment.

FIG. 3 shows a typical schematic spectral response of the TFPV device ofFIG. 2.

FIG. 4 shows a cross-sectional schematic view of a structure forming onesingle junction flexible TFPV device capable of bifacial lightcollection, in accordance with an embodiment.

FIG. 5 shows a flowchart illustrating one process for fabricating a TFPVdevice in substrate configuration, in accordance with an embodiment.

FIG. 6 shows a flowchart illustrating one process for fabricating a TFPVdevice in superstrate configuration, in accordance with an embodiment.

FIG. 7 shows a cross-sectional schematic view of materials forming oneTFPV device in superstrate configuration, in accordance with anembodiment.

DETAILED DESCRIPTION

The present disclosure may be understood by reference to the followingdetailed description taken in conjunction with the drawings brieflydescribed below. It is noted that, for purposes of illustrative clarity,certain elements in the drawings may not be drawn to scale.

TFPV solar cells based upon Group II-VI and Group I-III-VI.sub.2light-absorbing materials may provide low cost photovoltaic technologyand may be deposited on lightweight and flexible substrates for highefficiency (W/m²) and specific power (W/Kg) characteristics. Asemi-transparent back contact with a high-temperature, lightweight andflexible semi-transparent substrate may enable higher efficiency TFPVdevices, as compared to prior art devices, due to bifacial collection(e.g., above bandgap transmission into the device) and infrared (IR)light transmission out of the device (e.g., sub-bandgap).

A process is thus disclosed for forming a relatively low resistancesemi-transparent back contact between p-type Group I-III-VI.sub.2 orGroup II-VI solar absorbers and a Transparent Conducting Oxide (TCO),which can transmit sub-bandgap light and/or enable bifacial operation.This leads to a low cost, power efficient tandem TFPV device using asemi-transparent back contact formed by the process. Furthermore, a lowcost, power efficient single junction TFPV device for bifacial operationmay be produced using the semi-transparent back contact in conjunctionwith a semi-transparent, lightweight, and flexible substrate. Otherfeatures and advantages will become apparent in the following detaileddescription.

FIG. 1 shows a cross-sectional schematic view of a TFPV device 100 thatis fabricated with Group I-III-VI.sub.2 (e.g., CuInGaSe₂), Group II-VI(e.g, CdSe), or Group III-V (e.g., GaAs) solar absorber materials 113and a semi-transparent back contact 118, which is semi-transparent to atleast infrared light. TFPV device 100 also has a substrate 116, aheterojunction partner layer 112 (sometimes referred to as a windowlayer), an optional buffer layer 119 and a top contact 111. Back contact118 is shown with two layers: a semi-transparent contact interface 114;and a semi-transparent contact 115. Semi-transparent contact interface114 and semi-transparent contact 115 are at least partially transparentto infrared light. In embodiments, semi-transparent contact interface114 and semi-transparent contact 115 are also semi-transparent tovisible light. However, semi-transparent contact interface 114 may beomitted for low bandgap Group I-III-VI.sub.2 solar absorber basedbifacial devices where a high temperature and flexible substrate isused. Optional buffer layer 119 may, for example, represent an optional‘buffer’ layer of insulating ZnO employed in many CIGS devices.

As shown, TFPV device 100 is formed in a substrate configuration withsubstrate 116 located below semi-transparent back contact 118 (relativeto the direction of primary light incidence 120 upon a top surface 117of TFPV device 100). However, a superstrate configuration may beprovided for optically transparent substrates, by instead locatingsubstrate 116 above top contact 111, relative to the direction ofprimary light incidence 120, in accord with another embodiment andwithout departing from the scope hereof. Substrate 116 may be rigid orflexible. Substrate 116 may, for example, be formed of at least one ofglass, silicone, silicone resin, reinforced silicone, reinforcedsilicone resin, and high temperature polyimide.

Semi-transparent back contact layer 118 and substrate 116 may transmitsub-bandgap light 122 away from TFPV device 100 when the substrate issemi-transparent to sub-bandgap light, thereby reducing the operatingtemperature of TFPV device 100. Semi-transparent back contact layer 118and substrate 116 may also transmit above-bandgap light into the TFPVdevice 100, thereby enabling bifacial light collection or absorption.Both of these mechanisms can increase the device operating efficiency(W/m²), power output (W/kg), and output voltage (Voltage/cell).

Layers 111, 112, 113, 114 and 115 of TFPV device 100 may be used as atop cell of a tandem device (e.g., tandem device 200, FIG. 2).

The electro-optical properties of CuInTe₂ and CuAlTe₂ have beenconsidered for use in TFPV devices. More specifically, the electronaffinity of CuInTe₂ is about 0.5 eV less than CuInSe₂ (CIS), but thebandgap of the telluride remains about the same as the selenide. Undercertain circumstances, and considering theoretical band energies, thisindicates that the band-edge discontinuity between the valence band ofthe telluride and the work function level of traditional opaque Mo backcontacts may be 0.5 eV less than that of discontinuity of the selenideand traditional opaque Mo back contacts, which may result in lowercontact resistance (lower Schottky barrier height) for the telluridecompared to the selenide. Moreover, the telluride has the potential forbeing p-type and degenerately doped than selenide, which would also aidin the formation of a low resistance contact. This reasoning was thenapplied to the wider bandgap alloys with aluminum or CuAlSe₂ andCuAlTe₂. As a semi-transparent back contact interface layer 114, CuAlTe₂has good optical transparency in the wavelength region of interest sinceas it has a bandgap of 2.06 eV. The higher valence band-edge energy ofthe telluride may be responsible for the good contact with Mo and mayalso be helpful in forming contacts to TCOs.

Semi-transparent back contact 118 may, for example, be fabricated bydepositing semi-contact interface 114 onto semi-transparent contact 115.Semi-transparent contact interface 114 may, for example, be awide-bandgap alloy of Cu(X)(Te)₂ where X═In, Ga, or Al, or anycombination of these three elements that results in a lower resistancebetween semi-transparent contact 115 and solar absorber 113. Suchsemi-transparent contact interface formed of a wide-bandgap alloy ofCu(X)(Te)₂ is sometimes referred to as a CIGAT contact interface.Ideally, the bandgap of the Cu(X)(Te)₂ semi-transparent contactinterface 114 would also be selected to be much higher than the bandgapof the solar absorber 113 to maximize the transparency of above-bandgaplight to the solar absorber during bifacial operation. Alternatively,the semi-transparent contact interface layer 114 may be a thin (lessthan 100 Å, for example) metal layer (e.g., Mo) that is semi-transparentdue to the low thickness or incomplete coverage, and relatively inert tothe solar absorber deposition process. Semi-transparent contactinterface 114 may provide low absorption of light due to its lowthickness and/or incomplete coverage and/or its wide-bandgapcharacteristic. Semi-transparent contact 115 may, for example, beconductive electrodes consisting of TCOs such as ZnO:Al, Indium TinOxide (ITO), or SnO₂, or a similarly transparent conducting materialsuch as Stannates, or transparent layers with carbon nanotubes. Thisprocess of forming a low resistance and semi-transparent back contact118 to Group I-III-VI.sub.2 materials (e.g., solar absorber 113) for usein TFPV devices (e.g., TFPV device 100) allows sub-bandgap light to betransmitted through semi-transparent back contact 118, while alsoenabling bifacial operation of TFPV device 100. Other semi-transparentback contact interface layers 114 such as thin Cu, or Cu doped ZnTe,which are detailed in published papers or patents, may be appropriatefor CdTe based solar absorbers 113.

In one example of fabrication of TFPV device 100, a semi-transparentcurrent carrying transparent contact 115 is deposited onto substrate 116by means of sputtering, chemical vapor deposition, evaporation, or otherthin-film deposition technique. Semi-transparent contact interface 114,which may be formed of CuAlTe₂, may be likewise be deposited ontosemi-transparent contact 115 by means of sputtering, chemical vapordeposition, evaporation, or other thin-film deposition techniques.

A Group I-III-VI.sub.2 p-type material (e.g., solar absorber 113) may bedeposited onto semi-transparent back contact 118 with an optional n-typesemiconductor surface layer. Solar absorber 113 may optionally have anear surface region that is n-type. Deposition of solar absorber 113may, for example, be achieved by means of co-evaporation, thermalevaporation, spraying, printing, or other thin-film depositiontechniques and may contain selenides, sulfides, and tellurides of Cu,Ag, Al, Ga, In, Tl, and their alloys. In one example, solar absorber 113may be a variation of Cu(In, Ga, Al)(Se, S)₂ such as CIGS.

A heterojunction partner layer 112 may be deposited by chemical bathdeposition (CBD), chemical vapor deposition, sputtering, or other knowntechniques. Heterojunction partner layer 112 is, for example, CdS, ZnS,(Cd, Zn)S, ZnSe, ZnO, or SnO₂.

Buffer Layer 119, if included, may, for example, be deposited bychemical bath deposition (CBD), chemical vapor deposition, sputtering,or other technique.

Top contact layer 111 may be deposited onto heterojunction partner layer112 or buffer layer 119 and may be mostly transparent to the solarspectrum. In one example, top contact layer 111 is a TCO (e.g., ITO ordoped ZnO). A semi-transparent current carrying transparent contact 111is deposited onto the buffer or heterojunction partner layer by means ofsputtering, chemical vapor deposition or other thin-film depositiontechnique.

FIG. 2 shows a cross-sectional schematic view of a monolithic tandemdevice 200 having a top cell 218 containing a wide-bandgap solarabsorber layer 212 and a bottom cell 220 containing a low-bandgap solarabsorber 206 that are joined by a semi-transparent interconnect layer210. Top cell 218 is, for example, similar to TFPV device 100 in FIG. 1.Since top cell 218 may transmit sub-bandgap light, bottom cell 220 maybe designed and fabricated to absorb and convert this light toelectricity. Tandem device 200 may, for example, be based on thin-filmhigh-efficiency and low cost CIS and related alloys that provide ahigher efficiency (W/m²), specific power (W/kg), and Voltage(Voltage/cell) than do existing single-junction CIGS devices. Inaddition, tandem device 200 may also have a lower cost per unit poweroutput. In one embodiment, tandem device 200 is flexible (e.g., by usingflexible material layers and substrate). In another embodiment, tandemdevice 200 is rigid (e.g., glass substrate or other rigid substrate).

Tandem device 200 is shown with a substrate 202, a back contact 204, abottom solar absorber 206, a bottom heterojunction partner layer 208(sometimes referred to as a window layer), an optional buffer layer 222,a semi-transparent contact 213, a semi-transparent contact interface211, a top solar absorber 212, a top heterojunction partner layer 214,an optional buffer layer 219, and a top contact 216. Tandem device 200may, for example, utilize high-efficiency, low cost, and easy tomanufacture Group I-III-VI.sub.2 solar absorber materials, due to theability for bandgap engineering. An efficient wide-bandgap top solarabsorber 212 is desirable for top cell 218 as it is beneficial to theperformance of good tandem devices. Top cell 218 is formed uponsemi-transparent interconnect 210 (i.e., semi-transparent interconnect210 functions as a back contact for top cell 218). Semi-transparentinterconnect 210 may, for example, be similar to back contact 118 ofFIG. 1, and is shown with a semi-transparent contact interface 211 and asemi-transparent contact 213. Top contact 216, top heterojunctionpartner layer 214, top solar absorber 212, and semi-transparentinterconnect 210 form top cell 218. TFPV device 100 of FIG. 1 may beused (without substrate 116) as the top cell 218 of tandem device 200.

Top cell 218 of tandem device 200 may be capable of transmitting unusedabove-bandgap light and sub-bandgap light to bottom cell 220. Sinceabsorption of light that is not converted to electricity may increasethe operating temperature of top cell 218, as it may with any singlejunction device, allowing unused light to be transmitted to bottom cell220 may increase the efficiency of top cell 218.

FIG. 3 shows an illustration of an idealized spectral response graph 300of TFPV device 200 of FIG. 2. A top cell spectral response curve 302 oftop cell 218 has a spectral edge 306. A bottom cell spectral responsecurve 304 of bottom cell 220 has a spectral edge 308. Top cell spectralresponse 302 has a wide-bandgap, represented by spectral edge 306, andbottom cell spectral response 304 has a smaller bandgap, represented byspectral edge 308 and at a higher light wavelength than the wide-bandgapspectral edge.

FIG. 4 shows a cross-sectional schematic view of a structure forming oneexemplary embodiment of single junction flexible TFPV device 400 that iscapable of bifacial light collection. TFPV device 400 is fabricated upona semi-transparent substrate 402 and has a semi-transparent back contact408 (which may, for example, be formed of a semi-transparent contact 404and a semi-transparent contact interface 406 as shown), a solar absorber410 (e.g., Group I-III-VI.sub.2, Group II-VI, Group III-V), aheterojunction partner layer 412 (sometimes referred to as a windowlayer), an optional buffer layer 413, and a top contact 414. Sincesemi-transparent back contact 408 components of TFPV device 400 aresemi-transparent to visible and infrared light, bifacial lightcollection is possible, and heating that results from absorbing unusedlight is lowered.

The use of semi-transparent back contact 408 (also referred to herein as“transparent back contact 408” or “back contact 408”) and IR transparentsubstrate 402 may reduce the operating temperature of TFPV device 400.The reduction in operating temperature may increase the operatingefficiency (e.g., W/m²), specific power output (W/kg), and outputvoltage (Voltage/cell) of TFPV device 400. In addition, semi-transparentback contact 408 may enable bifacial operation of TFPV device 400 whensubstrate 402 is selected to be semi-transparent to above-bandgap light.In certain space or high altitude applications, bifacial operation mayincrease the output of TFPV device 400 by as much as 30%.

In FIG. 4, TFPV device 400 is shown in substrate configuration. However,layers 404, 406, 410, 412 and 414 may also be fabricated in asuperstrate configuration without departing from the scope hereof.

Substrate 402 may be lightweight, flexible, and be formed ofsemi-transparent materials. For example, substrate 402 may comprisesilicone or silicone resin. Substrate 402 may be reinforced, ifnecessary, as described in U.S. Provisional Patent Application Ser. No.60/792,852 and in U.S. Non-Provisional patent application Ser. No.11/737,119. Substrate 402 may be capable of withstanding processingtemperatures up to 600° C. thereby enabling use of high-efficiency CIGSand CdTe materials in fabricating TFPV device 400.

Semi-transparent contact 404 may be a semi-transparent current carryingTCO such as ZnO:Al, ITO, and SnO₂, or a similarly transparent conductingmaterials such as Stannates or transparent layers with carbon nanotubes.Semi-transparent contact interface 406 may be a thin semi-transparentlayer that is applied to semi-transparent contact 404 to lowerresistance between solar absorber 410 and semi-transparent contact 404.Semi-transparent contact 404 may be deposited by means of sputtering,chemical vapor deposition, or other thin-film deposition techniques.

In one embodiment, semi-transparent contact interface 406 is adiscontinuous layer of metal, and/or is a very thin semi-transparentlayer of metal, for use with CuInGaAlSe₂ (CIGAS) based solar absorbingmaterial. In another embodiment, semi-transparent contact interface 406is a wide-bandgap semiconductor fabricated from Cu(In, Ga, Al)(Te)₂ foruse with CIGS based solar absorbers or from ZnTe for use with CdTe basedsolar absorbers. Semi-transparent contact interface 406 may, forexample, be deposited by means of sputtering, chemical vapor deposition,co-evaporation, or other thin-film deposition techniques.

Solar absorber 410 may be a p-type semiconductor layer with an optionaln-type semiconductor surface layer formed from the copper, or group I,deficient phase of the solar absorber. Deposition of solar absorber 410may be by means of co-evaporation or other thin-film depositiontechniques.

Heterojunction partner layer 412 may be deposited by chemical bathdeposition (CBD) or other deposition techniques. The technique employed,however, should not damage the surface of solar absorber 410.Heterojunction partner layer 412 may, for example, be an n-typesemiconductor fabricated from either CdS, (Cd, Zn)S, ZnO, ZnOH, or ZnSe.

A top contact layer 414 makes contact with n-type heterojunction partnerlayer 412 and may be mostly transparent to the solar spectrum. In oneembodiment, top contact layer 414 is a transparent ITO or ZnO:Al.Optionally, buffer layer 413 may be included between top contact layer414 and heterojunction partner layer 412. Buffer layer 413 may be alayer of insulating ZnO as found in many CIGS devices.

TFPV device 400 may be further fabricated using monolithicinterconnection or by other means.

In one example of operation, incident light 418 (e.g., visible solarlight or white light) primarily enters the top 416 of TFPV device 400and passes through top contact layer 414. Light 418 is then partiallyabsorbed by heterojunction partner layer 412 and p-type solar absorber410. In particular, above-bandgap light is mostly absorbed by p-typesolar absorber 410 and sub-bandgap light is mostly transmitted throughsolar absorber 410, semi-transparent back contact 408, andsemi-transparent substrate 402, and away from TFPV device 400 (shown assub-bandgap light 420). Similarly, light 422 (e.g., reflected solar orwhite light) incident on the back of TFPV device 400 enters throughsemi-transparent substrate 402 and semi-transparent back contact 408,and above-bandgap light is then absorbed by solar absorber 410 andheterojunction partner layer 412. Below bandgap light 424 (i.e., unusedlight) is then mostly transmitted out through top contact 414, and awayfrom TFPV device 400.

FIG. 5 is a flowchart illustrating one example of a process 500 offabricating a bifacial TFPV device (e.g., TFPV device 400, FIG. 4). Asemi-transparent contact layer is deposited, in step 502, onto asemi-transparent substrate. In one example of step 502, semi-transparentcontact layer 404 is ITO and is deposited onto semi-transparentsubstrate 402 made of silicone resin. A thin semi-transparent contactinterface layer is deposited, in step 504, onto the semi-transparentcontact layer deposited in step 502. In one example of step 504, asemi-transparent contact interface layer 406 is a 1,000 Angstrom thicklayer of CuAlTe₂, with a Cu/Al ratio of 1, and is deposited ontosemi-transparent contact layer 404, which is ITO. A solar absorber layeris deposited, in step 506, onto the semi-transparent contact interfacelayer deposited in step 504. In one example of step 506, solar absorberlayer 410 is CuInGaAlSe₂, and is deposited onto semi-transparent contactinterface layer 406, which is CuAlTe₂.

In step 508, a transparent heterojunction partner layer is depositedonto the solar absorber layer deposited in step 506. In one example ofstep 508, heterojunction partner layer 412, is CdZnS and is depositedonto solar absorber layer 410, which is CuInGaAlSe₂. Step 509 isoptional. In step 509, a buffer layer is deposited onto the transparentheterojunction partner layer of step 508. In one example of step 509,buffer layer 413 is ZnO, and is deposited onto heterojunction partnerlayer 412, which is CdZnS. A semi-transparent top contact layer isdeposited, in step 510, onto the transparent heterojunction partnerlayer of step 508 (or the buffer layer of step 509, if included). In oneexample of step 510, top contact layer 414 is ITO and is deposited ontoheterojunction partner layer 412, which is CdZnS.

FIG. 6 shows a flowchart illustrating one process 600 for fabricating aTFPV device in superstrate configuration. FIG. 7 shows a cross-sectionalschematic view of materials forming one example of a TFPV device 700 insuperstrate configuration. FIGS. 6 and 7 are best viewed together withthe following description.

In step 602, a semi-transparent top contact layer 714 is deposited ontoa semi-transparent substrate layer 702. In one example of step 602, topcontact layer 714 is a TCO, such as ITO, that is sputtered ontosubstrate 702, which is made of silicone resin. Step 603 is optional. Instep 603, a buffer layer is deposited onto the semi-transparent contactlayer of step 602. In one example of step 603, buffer layer 713 is ZnO,and is deposited onto top contact layer 714, such as ITO. In step 604, atransparent heterojunction partner layer 712 (sometimes referred to as awindow layer) is deposited onto top contact layer 714 of step 602 oronto the buffer layer of step 603. In one example of step 604,heterojunction partner layer 712 is ZnO that is deposited onto topcontact layer 714, such as ITO, using a chemical bath depositiontechnique.

In step 606, a solar absorber layer 710 is deposited onto heterojunctionpartner layer 712. In one example of step 606, solar absorber layer 710is made of a Group I-III-VI.sub.2 p-type material, such as CIGS, whichis deposited using a thermal evaporation technique onto heterojunctionpartner layer 712. In another example of step 606, a Group II-VI or aGroup III-V material, such as CdSe, is deposited onto heterojunctionpartner layer 712. In step 608, a thin semi-transparent contactinterface layer 706 is deposited onto solar absorber layer 710. In oneexample of step 608, a wide-bandgap telluride based Group I-III-VI.sub.2p-type material, such as CuInGaAlTe₂, is deposited onto solar absorberlayer 710 using a sputtering technique. In step 610, a semi-transparentcontact layer 704 is deposited onto semi-transparent contact interfacelayer 706. In one example of step 610, a TCO, such as ITO, is depositedonto contact interface layer 706 using a sputter deposition technique.Layers 704 and 706 are illustratively shown together as back contactlayer 708 in FIG. 7.

As appreciated, the CIGAT semi-transparent contact interface layerdisclosed above may be used to improve contact to wide-bandgap solarabsorbers in PV devices that have non-transparent back contacts withoutdeparting from the scope hereof. For example, such devices may have aCIGAT semi-transparent contact interface layer adjacent to awide-bandgap solar absorber and have a thick opaque metal (e.g., Mo)adjacent to the CIGAT contact interface layer. Thus, TCO may not beneeded as the back contact if bifacial operation is not required. TheCIGAT semi-transparent contact interface layer may also be used with anopaque substrate (e.g., a silicone coated metal foil substrate) inembodiments of substrate configuration TFPV devices.

As appreciated, superstrate PV devices may use silicone or siliconeresin (reinforced or not) as the substrate and may have non-transparentback contacts, without departing from the scope hereof.

Experimental Results

Bifacial light collection testing was performed on TFPV device 100 or400 to determine if bifacial light collection occurs. AM1.5 light (100mW/cm²) from a solar simulator was incident on top of each device, whilea halogen lamp light source, also calibrated to 100 mW/cm², was directedtowards the substrate side of each device. Tests performed upon aCuInGaAlSe₂ (CIGAS) device with bandgap in the range 1.3 to 1.4 eV andfabricated on a glass substrate (substrate configuration) and using asemi-transparent back contact consisting of approximately 40 angstromthick molybedum contact interface layer on an ITO contact layer, showedthat an additional 25% power may be achieved through bifacial operation(due to a 14% increase in current and an improved fill factor) of thedevice. Transparency measurements of semi-transparent back contact 408on a glass substrate show a 40-60% transparency to visible light,indicating that there are additional reflectance losses and currentcollection losses within the device since only 25% increase in poweroutput is observed. Nonetheless, during operation in space, if about 30%of the space solar intensity were available to the back of the CIGASdevice (e.g., via albedo light), then an increase in output power ofover 8% (i.e., 30% of 25%) may be expected from this prototype device.Bifacial light collection testing of a CIGAS device fabricated upon alightweight and flexible silicone substrate, however, indicated only aslight increase in performance. It was determined that only a smallfraction of light was passing through back contact 408 and substrate402, possibly due to the thin metallic contact interface layer being toothick on the device tested. Thickness of semi-transparent contactinterface 406 may have a large effect on light transmission through backcontact 408. Nonetheless, enhanced performance with bifacial collectionwas demonstrated.

A comparison of non-bifacial device performance (Efficiency andOpen-circuit voltage) was performed on devices using lightweight andflexible silicone substrates and devices with similar composition onglass substrates for both opaque Mo back contacts and semi-transparentback contacts (bifacial capable) made from a very thin Mo contactinterface layer followed by an ITO transparent contact. The device wasin the substrate configuration and the solar absorber was low-bandgapCIGS. The non-bifacial testing results showed that the devices on thelightweight and flexible silicone substrates are capable of performingas well as devices on thick glass substrates, and likely even betterthan thick glass substrates if comparing just the results withsemi-transparent back contacts. Device efficiencies over 11% weredemonstrated with the traditional opaque Mo back contacts. Thus,lightweight and flexible substrates are compatible with the devicesdescribed herein and do not limit the CIGAS deposition temperature whichleads to higher performance devices.

In another comparison of non-bifacial device performance in thesubstrate configuration, two semi-transparent contact interface layerswere compared: a thin Mo layer, and a thin CuAlTe₂ layer. The solarabsorbers were high-bandgap CIGAS with [Ga+Al]/III ratios in the 54 to62 range, typically corresponding to bandgaps in the range of about1.55-1.65 eV. The semi-transparent contact layer for these devices ofthis comparison was florine-doped SnO₂, which together with thesemi-transparent contact interface layer and glass substrate, make thesedevices capable of bifacial collection. The 1.16 cm² devices wereilluminated from the top (through the top contact) with AM1.5 light (100mW/cm²) from a solar simulator and current-voltage measurements wereperformed. Three devices of each type were compared and the devices withthe CuAlTe₂ interface layers were identified as 70816-1A-C3,70822-1E-E3, and 70822-2A-05. The devices with the thin Mo interfacelayers were identified as 70816-1E-E3, 70822-1A-C3, and 70822-2E-E3. Thephotovoltaic device series resistance in the range of 2 to 2.5 volts wasused as the measure of back contact electrical performance, as in thisvoltage range other device effects are minimized. The series resistancefor the device with the CuAlTe₂ interface layer was found to be 26.3,5.8, and 19.2 ohm-cm² for an average of 17.1 ohm-cm². The seriesresistance for the device with the thin Mo interface layer was found tobe 6.1, 35.7, and 25.0 ohm-cm² for an average of 22.3 ohm-cm². Thesevalues are typical for the high-bandgap solar absorbers. As discussedabove, the thin Mo contact interface layers in combination with the TCOcontact layer have already shown comparable performance to thick opaqueMo layers when using high-bandgap solar absorbers. However the usablethin Mo contact interface layers have also shown to have limited visiblelight transmission, in the range of 60-70%, prior to the CIGASdeposition. Given that the CuAlTe₂ is a thin high-bandgap semiconductor,then the absorption loss through this layer is negligible compared tothe solar absorber itself.

Changes may be made in the above systems and processes without departingfrom the scope hereof. It should thus be noted that the matter containedin the above description or shown in the accompanying drawings should beinterpreted as illustrative and not in a limiting sense. The followingclaims are intended to cover all generic and specific features describedherein, as well as all statements of the scope of the present processand system, which, as a matter of language, might be said to fall therebetween.

1. A thin-film photovoltaic device, comprising: a semi-transparentsubstrate for supporting the thin-film photovoltaic device; asemi-transparent back contact layer disposed on the semi-transparentsubstrate, including: a semi-transparent contact layer disposed on thesemi-transparent substrate, and a semi-transparent contact interfacelayer including a Cu(X)Te₂ material disposed on the semi-transparentcontact layer, wherein X is at least one of In, Ga, and Al; a solarabsorber layer disposed on the semi-transparent back contact layer, thesolar absorber layer including one of a Group I-III-VI.sub.2 materialand a Group II-VI material; a heterojunction partner layer disposed onthe solar absorber layer; and a top contact layer disposed on theheterojunction partner layer.
 2. The thin-film photovoltaic device ofclaim 1, the semi-transparent contact layer comprising at least one of atransparent conductive oxide, a stannate, and a transparent layer withcarbon nanotubes.
 3. The thin-film photovoltaic device of claim 1, thesemi-transparent substrate comprising at least one of silicone, siliconeresin, reinforced silicone, and reinforced silicone resin.
 4. Thethin-film photovoltaic device of claim 1, the semi-transparent substratecomprising polyimide.
 5. The thin-film photovoltaic device of claim 1,the semi-transparent substrate comprising glass.
 6. The thin-filmphotovoltaic device of claim 1, the solar absorber layer comprising atleast one of a surface and a near surface region that is n-type.
 7. Thethin-film photovoltaic device of claim 1, further comprising a bufferlayer disposed between the heterojunction partner layer and the topcontact layer.
 8. A thin-film photovoltaic device, comprising: asemi-transparent substrate for supporting the thin-film photovoltaicdevice; a top contact layer disposed on the semi-transparent substrate;a heterojunction partner layer disposed on the top contact layer; asolar absorber layer disposed on the heterojunction partner layer, thesolar absorber layer including one of a Group I-III-VI.sub.2 materialand a Group II-VI material; and a semi-transparent back contact layerdisposed on the solar absorber layer, including: a semi-transparentcontact interface layer including a Cu(X)Te₂ material disposed on thesolar absorber layer, wherein X is at least one of In, Ga, and Al, and asemi-transparent contact layer disposed on the semi-transparent contactinterface layer.
 9. The thin-film photovoltaic device of claim 8, thesemi-transparent contact layer comprising at least one of a transparentconductive oxide, a stannate, and a transparent layer with carbonnanotubes.
 10. The thin-film photovoltaic device of claim 8, thesemi-transparent substrate comprising at least one of silicone, siliconeresin, reinforced silicone, and reinforced silicone resin.
 11. Thethin-film photovoltaic device of claim 8, the semi-transparent substratecomprising polyimide.
 12. The thin-film photovoltaic device of claim 8,the semi-transparent substrate comprising glass.
 13. The thin-filmphotovoltaic device of claim 8, the solar absorber layer comprising atleast one of a surface and a near surface region that is n-type.
 14. Thethin-film photovoltaic device of claim 8, further comprising a bufferlayer disposed between the heterojunction partner layer and the topcontact layer.
 15. The thin-film photovoltaic device of claim 8, furthercomprising a bottom photovoltaic device disposed on the semi-transparentcontact layer of the semi-transparent back contact layer, the bottomphotovoltaic device being operable to absorb sub-bandgap light passingthrough the semi-transparent back contact layer.
 16. The thin-filmphotovoltaic device of claim 15, the bottom photovoltaic devicecomprising: a bottom heterojunction partner layer disposed on thesemi-transparent contact layer of the semi-transparent back contactlayer; a bottom solar absorber layer disposed on the bottomheterojunction partner layer; and a bottom back contact layer disposedon the bottom solar absorber layer.
 17. A thin-film photovoltaic device,comprising: a semi-transparent substrate for supporting the thin-filmphotovoltaic device; a semi-transparent back contact layer disposed onthe substrate, including: a semi-transparent contact layer disposed onthe semi-transparent substrate, and a semi-transparent contact interfacelayer disposed on the semi-transparent contact layer, thesemi-transparent contact interface layer including at least one of asemi-transparent metal layer having a thickness of less than 100angstroms and a discontinuous metal layer; a solar absorber layerdisposed on the semi-transparent back contact layer; a heterojunctionpartner layer disposed on the solar absorber layer; and a top contactlayer disposed on the heterojunction partner layer.
 18. The thin-filmphotovoltaic device of claim 17, the semi-transparent substratecomprising at least one of silicone, reinforced silicone, and reinforcedsilicone resin.
 19. The thin-film photovoltaic device of claim 17, thesolar absorber layer comprising at least one of a group I-III-VI.sub.2material and a group II-VI material.
 20. The thin-film photovoltaicdevice of claim 17, the semi-transparent metal layer having a thicknessof less than 100 angstroms comprising Molybdenum.
 21. The thin-filmphotovoltaic device of claim 17, the semi-transparent substratecomprising polyimide.
 22. The thin-film photovoltaic device of claim 17,the semi-transparent substrate comprising glass.
 23. A thin-filmphotovoltaic device, comprising: a semi-transparent substrate forsupporting the thin-film photovoltaic device; a top contact layerdisposed on the semi-transparent substrate; a heterojunction partnerlayer disposed on the top contact layer; a solar absorber layer disposedon the heterojunction partner layer; and a semi-transparent back contactlayer disposed on the solar absorber layer, including: asemi-transparent contact interface layer disposed on the solar absorberlayer, the semi-transparent contact interface layer including at leastone of a semi-transparent metal layer having a thickness of less than100 angstroms and a discontinuous metal layer, and a semi-transparentcontact layer disposed on the semi-transparent contact interface layer.24. The thin-film photovoltaic device of claim 23, the solar absorberlayer comprising at least one of a group I-III-VI.sub.2 material and agroup II-VI material.
 25. The thin-film photovoltaic device of claim 23,the semi-transparent substrate comprising at least one of silicone,reinforced silicone, and reinforced silicone resin.
 26. The thin-filmphotovoltaic device of claim 23, the semi-transparent substratecomprising polyimide.
 27. The thin-film photovoltaic device of claim 23,the semi-transparent substrate comprising glass.
 28. The thin-filmphotovoltaic device of claim 23, the semi-transparent metal layer havinga thickness of less than 100 angstroms comprising Molybdenum.